U.S. Pat. No. 6,400,100 has already disclosed an operating method for the resonant operation of high pressure lamps in longitudinal mode, and an associated system and EB. Specified there is a method for finding the second longitudinal acoustic resonant frequency. It assumes that during the continuous sweeping of the frequency exciting the longitudinal mode the resonant frequency in the vertical operating position can be found by an occurrence of a relative increase in the operating voltage of the lamp. It emerges that the longitudinal frequency for a segregated arc state is found in vertical resonance with the aid of this method, and then maintained. However, depending on the composition of the metal halide filling and the timing of the expiry of the search procedure, this frequency thus found can be established far too high, and so an excitation of the acoustic resonance at the frequency found with the aid of the abovementioned method produces inadequate mixing and does not eliminate the segregation satisfactorily. Moreover, it is expensive to implement in an electronic ballast.
Otherwise, in previously known metal halide lamps the color temperature is fixed for an operating point exclusively by the lamp geometry, shape of the internal volume of the discharge vessel, electrode spacing and filling (buffer gas+selection of the metal halides 25) for a narrow power range Pn±3%. Possible in this case as type of operation for color-locus control is power variation or, given square-wave or a.c. operation also a control of the duty cycle, use being made of cataphoresis processes.